1. Field of the Invention
The present invention relates to variable capacity compressor for compressing a fluid such as a refrigerant in a vehicular air-conditioning system.
2. Description of the Related Art
As described for example in Japanese Unexamined Patent Publication (Kokai) No. 1-190972 and Japanese Unexamined Patent Publication (Kokai) No. 2-49982, in a variable capacity compressor for an air-conditioning system installed in a vehicle in the past, the temperature inside the vehicle has been kept constant by detecting the suction pressure of the refrigerant or the exhaust temperature of the cold air, while the suction pressure or exhaust temperature have been kept constant by changing the capacity of the compressor (discharge capacity, that is, amount of discharge per revolution of the drive shaft or per unit time) by a capacity control valve. Recently, however, to improve the fuel economy of the engine and the drivability of the vehicle, there have been strong demands for controlling the capacity of the compressor from the engine side or the vehicle side in accordance with the operating state of the engine or the running state of the vehicle.
To meet with these demands, in another prior art, for example, a capacity control device of a variable capacity compressor described in Japanese Unexamined Patent Publication (Kokai) No. 5-87048, in addition to such a capacity control valve, a solenoid valve operating by an electrical signal input from the outside is provided. Small capacity operation is forced from the outside through this solenoid valve so as to prevent a sharp rise in the torque at the time of startup of the compressor and thereby reduce the shock given to the vehicle. Further, in the method of control of a variable capacity compressor described in Japanese Unexamined Patent Publication (Kokai) No. 1-45978, a means has been proposed for changing the set suction pressure of the capacity control valve from the outside using a solenoid valve etc. so as to reduce the capacity of the compressor and lighten the load of the engine at the time of vehicle acceleration etc.
The problem common to these prior art is that the capacity control valve used is complicated in structure and therefore becomes large in size. Further, due to the same reason, the cost of the compressor rises or the compressor as a whole becomes larger, so a large space is required in the engine compartment of the vehicle for installing the compressor. Further, in the prior art, since the magnitude of the torque generated due to the operation of the compressor was not known, the engine could not be operated under the optimal conditions, so the fuel economy of the engine could not be sufficiently improved. Alternatively, the capacity of the compressor could not be freely controlled in accordance with the running state of the vehicle, so when the engine load became larger such as during acceleration of the vehicle or when climbing a slope, the capacity of the compressor could not be made smaller. Therefore, the effect of control of the compressor in improving the drivability of the vehicle could not be sufficiently raised.